Conventionally, it has been known that metallic nano particles (or metallic colloidal particles) with a great specific surface area and high reaction activity have a property to be fused at a low temperature (low-temperature sintering) compared to bulk and metallic atoms, and an application to various fields, for example, as an conductive paste is expected by utilizing this property.
For example, in Patent Literature 1 (Japanese Patent Application Laid-Open No. 2009-097074), Patent Literature 2 (Japanese Patent Application Laid-Open No. 2011-058092) and Patent Literature 3 (Japanese Patent Application Laid-Open No. 2010-202943), a metallic nano particle paste forming a minute wiring pattern on a substrate is proposed.
In Patent Literature 1 above, a metallic nano paste that is coated with a protective colloid composed of amines and carboxylic acid with 4 or greater of carbon number is disclosed, and although it is considered that it is possible to form a pattern at a low temperature and to sinter the paste at a low temperature, at 150° C. or less, but there is still room to improve the dispersing stability of the metallic nano particle paste.
Further, in Patent Literature 2 above, a process where a metallic compound is reacted with a reductant in a substantially solvent-free reacting mixture containing the metallic compound, the reductant and a stabilizer, such as organic amine, in the presence of the stabilizer, and a plurality of metal-containing nano particles having a molecule of the stabilizer on the surface due to a solvent-free reduction process is disclosed, and it is considered that the obtained metallic nano particle has a long storage life (stability) and a low annealing temperature, but an evaluation method for stability is only according to visual examination, and there is still room to improve the dispersing stability.
In addition, in Patent Literature 3 above, an aggregate of metallic colloid particles containing metallic nano particles and a dispersant is disclosed, and the dispersant is composed of a coagulation aid having at least one type of functional group selected from a group constituting a group having a nitrogen atom, a hydroxyl group and a carboxyl group; and a polymer dispersant, and its purpose is to collect and agglutinate particles, but the dispersing stability is not considered.
Further, the metallic nano particles (or metallic colloidal particles) as mentioned above are expected to be utilized for a use application to bonding of metallic parts due to their low-temperature sinterability. Since the sintered metallic nano particles (or metallic colloidal particles) indicate a similar characteristic to the same type of bulk materials, a junction part having very high heat resistance, reliability and heat dissipation can be obtained.
For example, in Patent Literature 4 (Japanese Patent Application Laid-Open No. 2011-094223), a bonding material for inorganic materials composed of metallic nano particles, a dispersant and a solvent is disclosed, and an inorganic material can be solidly joined under bonding conditions at a low temperature and at low pressure by controlling the particle size of the metallic nano particle to be 50 nm or less and 100 nm to 200 nm. However, in order to obtain metallic nano particles having wider particle size, a use of a coagulation aid is required, and compatibility between dispersing stability and high strength has not been accomplished.
Further, in Patent Literature 5 (Japanese Patent Application Laid-Open No. 2006-252976), a metallic nano paste that is obtained by adding an organic solvent with 185° C. to 250° C. of boiling point into a solid sol, which is obtained from a metallic nano particle solution with 93% by weight or greater of metallic concentration in solid content is disclosed. However, the objective is to prepare a metallic nano paste that is preferably usable for formation of an conductive pattern using screen printing by optimizing the metallic concentration, a solvent additive amount and viscosity, and the metallic nano paste is not considered as a composition for metal bonding at all.